This invention relates to a communication system and, particularly, to a communication system for converting linear delta modulated (hereafter LDM) signals into compressed pulse code modulated (hereafter CPCM) signals and to a system for converting CPCM signals into LDM signals.
The demand for communication services has been steadily increasing. In meeting this demand, it has proved effective in some communication systems to convert analog signals presented to the system into digital signals, transmit the digital signals, and then reconvert the digital signals into analog signals corresponding to those originally input into the system. One example of a communication system in which such digital conversion of analog signals has proved to have particular utility is a telephone communication system.
In telephone systems, it is known to periodically sample a continuous input analog signal, such as a voice signal and to create a digital signal of an encoded series of pulses representing by its encoding the input analog signal at each instant of sampling. The digital signal is transmitted and then decoded into an analog signal corresponding to that originally input into the system. Several schemes for so digitally encoding analog signals are known.
In one encoding scheme, the amplitude of a continuous analog signal is periodically sampled and each sample converted into a digitally encoded pulse sequence or word representing a quantum of analog signal amplitudes including that of the signal sample. This operation is called sampling and quantizing the analog signal. If the quantum levels or steps of the quantizing operation are uniform for all analog signal amplitudes, the encoded signal is said to be linear pulse code modulated (hereafter LPCM). Each LPCM signal word may then be decoded to form an analog signal of an amplitude substantially corresponding to the amplitude of the analog signal originally input into the system and encoded into the word. Since the continuous input analog signal was periodically sampled, the resulting periodic series of such LPCM signal words may be formed into a continuous analog signal substantially corresponding to the continuous input analog signal.
In the quantizing process, the exact level of the analog input signal at the sampling instant is, as described, approximated by one of a number of discrete values or quantum levels digitally encoded as the LPCM signal. The difference between the instantaneous amplitude of the input analog signal and the quantum level actually transmitted is called quantizing error and gives rise to what is variously known as quantizing noise or quantizing distortion.
Quantizing distortion is especially objectionable and very often intolerable when the instantaneous amplitude of the input analog signal is small, but is usually of little or no significance when the instantaneous amplitude of the input analog signal is high because the low amplitude of the input signals permits a relatively low level of quantizing noise to significantly degrade the ratio of signal to noise while a higher amplitude of the input signal can tolerate greater quantizing noise within an acceptable ratio of signal to noise. It is therefore desirable to have smaller quantum levels for lower amplitudes of the input signal to achieve closer correspondence between the quantum level of the encoded signal and the actual input analog signal at lower amplitudes than for higher amplitude input signals. Of course the size of the quantum levels for all input signal amplitudes could be decreased by this produces an undesirable increase in the total number of quantum levels, requiring, for example, more binary bits to represent the signal as a digitally encoded word.
The suggested non-linear redistribution of the size of the quantizing levels is called companding, a verbal contraction of the terms compression and expanding. The purpose of companding is then to reduce the quantizing impairment of the original signal without unduly increasing the total number of quantizing levels by quantizing on a non-linear rather than a linear basis.
It is current practice with telephone systems to compand encoded analog signals on either a "mu-law" or an "A-law" companding scheme as described by H. Kaneko in an article entitled "A Unified Formulation of Segment Companding Laws and Synthesis of Codecs and Digital Companders," Bell System Technical Journal, September, 1970. Both these laws define segments or chords of a piecewise linear curve generally exponentially increasing for increasing levels of input analog signal amplitude. Each chord is divided into an equivalent number of quantization steps defining between them the intervals or quantization levels into which the analog signal will be encoded. The companding encoding scheme is then to encode each sampled analog signal amplitude into a combined sequence of two encoded signals, one representing the chord generally corresponding to the analog signal amplitude and the other representing the step along the identified chord more precisely corresponding to the analog signal amplitude. The resulting signals are then called compressed pulse code modulated signals (hereafter CPCM) or companded pulse code modulated signals. One device for so encoding input analog signals is disclosed in co-pending U.S. Pat. application Ser. No. 385,095 filed Aug. 2, 1973 in the names of Wintz, Sergo and Song. Of course, the CPCM signals may also be decoded into an analog signal. One device for so decoding CPCM signals is disclosed in co-pending U.S. Pat. applicaton Ser. No. 402,342 filed Oct. 1, 1973 in the names of Wintz and Sergo.
Still another scheme for encoding analog signals periodically samples the analog signal and compares the amplitude of the signal at each sampling instant with a signal representing the predicted amplitude of the analog signal from the immediately preceding sampling instant to form a binary-encoded signal from the comparitor indicating by its one of the two possible binary states whether the instant sample of the analog signal is greater or less than the sample at the preceding instant. In general, the signal from the comparator is integrated to locally generate a signal representing the amplitude of the analog signal at the preceding sampling instant for comparison in the comparator with the instantaneous sample of the analog signal. Then, for example, if the analog signal is greater at one sampling instant than the locally generated signal representing the amplitude of the analog signal at the immediately preceding sampling instant, the comparator provides a high logic level signal, and, if the signal is less than the locally generated signal, the comparator provides a low logic level signal. Such binary-encoded signals are called linear delta modulated (hereafter LDM) signals.
The effectiveness of such LDM signals in representing analog signals largely depends upon the accuracy of the locally generated signal representing a preceding sample of the analog signal. It has been shown that the relative accuracy of the locally generated signal may be maximized by keeping the sampling rate high and the increments or quantizing steps in locally generating the signal representing the preceding analog signal relatively small, to thereby provide a large number of LDM signals integrated to closely approximate the input signal so that the quantizing error in encoding an individual LDM signal will not represent a substantial excursion of the LDM signal from the actual analog input signal. Unfortunately, the sampling rates required to achieve the same quality or signal to noise ratio and dynamic range from such LDM signals in comparison to a similar signal encoded in a 7-bit "mu-law" CPCM scheme is 19.6 MHz and, in an 8-bit scheme, 39.2 MHz, frequencies substantially at the limit of modern digital technology. At the same time, attempts to provide variable size integration steps to the integrator, usually called adaptive delta modulation, introduce additional complication to delta modulation equipment thereby reducing the attractiveness of its theoretical simplicity. One example of an adaptive delta modulation device is disclosed in U.S. Pat. No. 3,652,957 issued Mar. 28, 1972 in the name of Goodman, but it requires high speed LDM signal encoding devices to provide high quality adaptive delta modulation signals. These problems have inhibited the commercial use of delta modulation encoding schemes.
In addition, the CPCM encoding scheme was first developed and, generally in a 6 and 7-bit plus sign bit format, placed in substantial commercial use in telephone systems. It is obviously desirable to have every portion of a telephone system compatible with every other portion of the system to permit the interchange of signals between every portion of the telephone system. Given the large number of telephone systems already in use with the CPCM encoding scheme, it is economically unfeasible to eliminate all telephone equipment utilizing the CPCM encoding scheme and substitute equipment utilizing a delta modulation encoding scheme. This problem has also inhibited the use of delta modulation.
Nevertheless, the relative simplicity of the LDM encoding scheme makes desirable the use of this scheme in telephone equipment, particularly telephone equipment between a subscriber and a central office which generally is not now digitally encoded. As is apparent from a mere description of the encoding scheme in which a CPCM signal has been described as a two-section, nonlinearly encoded digital word and an LDM signal described as a single binary-encoded digit, a delta modulating scheme offers substantial simplicity of equipment necessary for its implementation in comparison to a CPCM encoding device. Given the large number of telephone subscribers, the simplicity and thus lower cost of delta modulation equipment offers economic attraction for digitally encoding signals between each telephone subscriber and means processing signals from the subscriber. Therefore, there is a desire to introduce delta modulation devices into a telephone system.
In order to achieve the desired compatibility between such delta modulated portions of a telephone system and portions already utilizing a CPCM encoding scheme it is then necessary to reversibly convert signals between the LDM and CPCM signal encoding schemes. One such conversion device is disclosed in related U.S. Pat. Nos. 3,707,712 issued Dec. 26, 1972 and 3,772,678 issued Nov. 13, 1973, both in the names of Deschenes and Villeret. Another device for conversion of analog to linear delta modulated signals, and then to pulse code modulated signals is disclosed by D. J. Goodman in an article titled "The Application of Delta Modulation to Analog-to-PCM Encoding in The Bell System Technical Journal," Vol. 48, No. 2, February 1969, Pages 321-343.